1. Field of the Invention
The present invention relates to an electrode including a Si-containing material layer and a porous layer, and a lithium battery employing the same, and more particularly to an electrode, in which a Si-containing material is applied on an electrode current collector and/or an electrode active material, to protect the surface of the electrode current collector from oxidation, and to enhance the adhesion between the electrode current collector and the electrode active material, leading to improvement in cycle life characteristics, and to increase the adhesion between the electrode active material and the porous film, thus reducing resistance, improving ohmic contacts and lowering the Shottkey barrier, and which includes a porous film functioning as a separator, and thus can provide a battery which is safe under conditions of overcharge and heat exposure without needing an additional separator, as well as a lithium battery employing the electrode.
2. Description of the Prior Art
These days, compact and lightweight electrical/electronic devices such as portable phones and notebook computers are being actively developed and produced. Such portable electrical/electronic devices include battery packs such that these devices can be operated without a separate power supply. Such battery packs have at least one battery, and recently, a rechargeable secondary battery has been used in the battery pack in terms of economy. Secondary batteries typically include nickel cadmium (Ni—Cd) batteries, nickel-hydrogen (Ni—H) batteries, and lithium secondary batteries such as lithium (Li) batteries and lithium ion (Li ion) batteries.
Particularly, the lithium secondary batteries are rechargeable through the absorption and release of lithium and can easily reach small size and large capacity. Also, these batteries have initially been the subject of many studies in terms of that the operation voltage thereof is three times as high as nickel-cadmium batteries or nickel-hydrogen batteries and have high energy density per unit weight. However, when a lithium metal is used in a negative electrode, a lot of dendrites grow on the lithium surface when charged, leading to a reduction in charge/discharge efficiency and short-circuits between the electrodes. Another problem is the instability (high reactivity) of lithium itself.
In an attempt to solve these problems, studies to use a carbon material in a negative electrode have been conducted. This kind of negative electrode is disclosed in, for example, Japanese Patent Publication Nos. Hei 5-299073, Hei 2-121258 and Hei 7-335623. In these studies, expansion or shrinkage caused by charge/discharge is reduced compared to the case of using the lithium or lithium alloy, but there are problems in that capacity is reduced and initial charge/discharge efficiency is lowered, compared to the case of using lithium.
For this reason, studies to increase the capacity of batteries by introducing metal such as lithium into a negative electrode have been actively attempted, however, these studies have conducted to increase electrical capacity while avoiding problems such as short circuits by suitably mixing lithium or lithium alloy with a carbon-based material, in view of problems such as the deposition of dendritic lithium and a rapid change in capacity, which occur when the metal such as lithium or alloy thereof is used alone. Regarding the use of these composite materials, Japanese Patent Publication No. 1993-286763 discloses a negative electrode material obtained by mixing a carbon-based material with a metal material of a size similar to that of the carbon material, coating the mixture with an organic compound and calcining the coated material. Also, Japanese Patent Publication No. Hei 6-349482 discloses a method capable of suppressing a rapid reduction in capacity even in high-rate discharge by adding a metal as a conductive agent to carbon for use in a negative electrode or positive electrode active material to reduce the contact resistance between the active materials or to reduce the contact resistance between an electrode current collector and the active material.
However, the structure, in which the electrode active materials are in direct contact with the electrode current collector, has problems in that, due to a difference in the ohmic contacts between a portion, in which the electrode active materials are in contact with the electrode current collector, and another portion, in which the electrode active materials are not in contact with the electrode current collector, a difference in the concentration of electron density occurs, so that the ionization of the electrode current collector progresses, and thus the resistance of the battery increases due to the oxidation of metal, and the ionization easily occurs even at low voltage. Also, it has a problem in that, at a portion in which an electric field line is not concentrated, ions of the electrode current collector are deposited as metals, causing dendritic phenomena.
For this reason, there is a need to solve problems associated with an increase in resistance, a reduction in ohmic contacts, a reduction in cycle life capacity and the like caused by the ionization of the electrode current collector, and to increase adhesion.
Also, in lithium ion batteries, an electrolyte has low ion conductivity. The low ion conductivity of the electrolyte acts as a factor of increasing the internal impedance of the battery to increase the internal voltage drop and of limiting the current and output of the battery, particularly when large current discharged is required.
Furthermore, a separator also acts as a factor of limiting the migration of lithium ions between two electrodes, whereas the separator of the lithium ion battery also functions as a safety factor of preventing the overheating of the battery by itself. If the separator reaches a given temperature or higher due to the abnormalities of the battery, a polyolefin-based porous film, which is a general material forming the separator, will be softened and partially melted. Thus, the micropores of the microporous film, which serve as passages for electrolyte solutions and lithium ions, will be shut down. When the flow of lithium ions stops, the current flow between the internal and external portions of the battery becomes blocked, thereby slowing or stopping the temperature increase in the battery. However, in a high-capacitance secondary battery, a large amount of current can flow over a short period of time. When excessive current flows in such a battery, the temperature in the battery cannot be decreased by shutting down the pores of the separator and blocking current flow. Furthermore, the heat generated by such excessive current flow may cause the separator to continue to melt and destruct. As a result, a short circuit due to the destruction of the separator becomes increasingly possible.
In these circumstances, although blocking the current flow by shutting down the pores of the separator is also important, a countermeasure against melting and contraction of the separator is further important to prevent the battery from overheating. In other words, it is required to stably prevent an internal short-circuit between electrodes even at high temperatures of, for example, 200° C. or higher.
Therefore, there is a need for the development of a lithium-ion secondary battery in which lithium ions can smoothly migrate and which is safe even at high temperatures.